Next, in order to achieve a neutral color balance in the sky background, the gamma needs to be adjusted for each of the three (red, green, blue) color channels independently.

Bring up the Levels tool again, and select Red in the Channel drop down. Click the gray-point (gamma) sampling icon (the middle icon below the Options button), then click in the image at a point that is part of the sky background. You can refer to other published images of the subject to determine where a good sky background point would be if necessary. In this image, the only area of sky background is found in the upper right corner - the rest of the image contains nebulosity. Repeat this for the other two color channels, then click OK.